Multiobject Spectroscopy of Young Star Clusters in NGC 4676

Abstract

Galaxy interactions are known to trigger starbursts. The young star clusters formed in mergers may be young globular clusters. The ages of these young star clusters yield the timing of interaction-triggered star formation and provide an important way to reconstruct the history of merging galaxies. Here we present the first results from our investigation into age and metallicity of 12 young clusters in the merging galaxy pair NGC 4676, using spectra from the multiobject Low Resolution Imaging Spectrometer (LRIS) on Keck. For 10 clusters, comparison of the Balmer emission lines with model equivalent widths (EWs) yields ages less than 10 Myr. Two spectra display Balmer absorption lines typical of star clusters dominated by A-type stars, with estimated ages of about 170 Myr. These ages are comparable to the dynamical age of the tidal tails and are consistent with star formation triggered during the first passage of the pair. The locations of these two clusters in the tidal tails are generally consistent with predictions of shock-induced star formation models. One of these older objects appears unresolved on the image and is luminous enough to qualify as a young globular cluster. Using EWs of the diagnostic lines [O II] and [O III], we obtain oxygen abundances in the range 7.3<12+log(O/H)<9.0. These values show a nearly flat distribution along the northern tail, suggesting efficient gas mixing in the tail.

The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, University of California, and National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W. M. Keck Foundation.